1,721,159 research outputs found
Rendezvous maneuvers of multiple spacecraft using differential drag under J2 perturbation
No full textIn this work the residual atmospheric drag is exploited to perform rendezvous maneuvers among multiple spacecraft in low Earth orbits. These maneuvers are required, for instance, for autonomous on-orbit assembly. By varying the level of aerodynamic drag of each spacecraft, relative differential accelerations are generated among the spacecraft of the group, and, therefore, their relative orbits are controlled. Each of the spacecraft is assumed to include a drag plate, which can be actively opened or closed, in order to vary the atmospheric drag. The recently developed Schweighart-Sedwick model is used to describe the relative dynamics of different spacecraft with respect to a circular orbit with the inclusion of J2 effects. Furthermore, the natural relative dynamics of each chaser with respect to the target is decoupled into a secular motion and a periodic oscillation. In particular, the following two-phase control method is proposed. First the secular motion of each chaser is controlled via differential drag in order for the spacecraft to sequentially move from an arbitrary initial condition to a closed stable relative orbit around the target spacecraft. After the relative orbit stabilization, a relative eccentricity control is applied to each spacecraft in order to zero out the semi-axis of the relative orbit around the target and achieve the rendezvous condition. The control algorithm considers mutual constraints among the values of differential drag that the different spacecraft can experience. Potential collisions are avoided by changing the maneuvering initial time. The main advantage of the proposed technique is that it enables a fleet of spacecraft to rendezvous without propellant expenditure. Furthermore, no numerical optimization is needed, as the control policy is based on closed-form analytical solutions. The proposed technique was validated via numerical simulations. © 2008 by the Authors
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Comparison of four natural pixel bases for SPECT imaging
No full textIn this paper the problem of restoring a two-dimensional tomographic medical image is considered. The emission function f (P )
is defined on a plane circular domain , and has to be restored from data collected by a SPECT machine. Through an approach
known as natural pixel discretization, the solution is expressed as a linear combination of functions belonging to a suitable basis.
We consider here four different bases, all of them giving a highly structured coefficient matrix. The linear system obtained in this
way can be solved efficiently by means of the fast Fourier transform. The computational cost and the performance of the bases are
compared. When the data are contaminated by Poissonian noise, the numerical experimentation shows that all the bases are almost
equivalent from the point of view of the restoration efficiency. Hence the choice of a basis should rely on other considerations, as
for instance the computational cost
Disagio e Benessere: alla ricerca dei mediatori fisiologici.
No full textThe aim of the present study is to describe the relationship between psychological well-being, ill-being and biological correlates on a sample of Italian blood donors. Particularly, specific correlations with biological correlates were analysed
Uncooperative Objects Pose, Motion and Inertia Tensor Estimation via Stereovision
Full text linkAutonomous close proximity operations are an arduous and attractive problem in space mission design. In particular, the estimation of pose, motion and inertia properties of an uncooperative object is a challenging task because of the lack of available a priori information. In addition, good computational performance is necessary for real applications. This paper develops a method to estimate the relative position, velocity, angular velocity, attitude and inertia properties of an uncooperative space object using only stereo-vision measurements. The classical Extended Kalman Filter (EKF) and an Iterated Extended Kalman Filter (IEKF) are used and compared for the estimation procedure. The relative simplicity and low computational cost of the proposed algorithm allow for an online implementation for real applications. The developed algorithm is validated by numerical simulations in MATLAB using different initial conditions and uncertainty levels. The goal of the simulations is to verify the accuracy and robustness of the proposed estimation algorithm. The obtained results show satisfactory convergence of the estimation errors for all the considered quantities. An analysis of the computational cost is addressed to confirm the possibility of an onboard application. The obtained results, in several simulations, outperform similar works present in literature. In addition, a video processing procedure is presented to reconstruct the geometrical properties of a body using cameras. This method has been experimentally validated at the ADAMUS (ADvanced Autonomous MUltiple Spacecraft) Lab at the University of Florida
The Sylvester-Kac matrix space
No full textThe Sylvester–Kac matrix is a tridiagonal matrix with integer entries and integer eigenvalues that appears in a variety of applicative problems. We show that it belongs to a four dimensional linear space of
tridiagonal matrices that can be simultaneously reduced to triangular form. We name this space after the matrix
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Multiple spacecraft rendezvous maneuvers by differential drag and low thrust engines
No full textA novel two-phase hybrid controller is proposed to optimize propellant consumption during multiple spacecraft rendezvous maneuvers in Low Earth Orbit. This controller exploits generated differentials in aerodynamic drag on each involved chaser spacecraft to effect a propellant-free trajectory near to the target spacecraft during the first phase of the maneuver, and then uses a fuel optimal control strategy via continuous low-thrust engines to effect a precision dock during the second phase. In particular, by varying the imparted aerodynamic drag force on each of the chaser spacecraft, relative differential accelerations are generated between each chaser and the target spacecraft along two of the three translational degrees of freedom. In order to generate this required differential, each chaser spacecraft is assumed to include a system of rotating flat panels. Additionally, each chaser spacecraft is assumed to have continuous low-thrust capability along the three translational degrees of freedom and full-axis attitude control. Sample simulations are presented to support the validity and robustness of the proposed hybrid controller to variations in the atmospheric density along with different spacecraft masses and ballistic coefficients. Furthermore, the proposed hybrid controller is validated against a complete nonlinear orbital model to include relative navigation errors typical of carrier-phase differential GPS (CDGPS). Limitations of the proposed controller appear relative to the target spacecraft's orbit eccentricity and a general characterization of the atmospheric density. Bounds on these variables are included to provide a framework within which the proposed hybrid controller can effect an extremely low propellant rendezvous of multiple chaser spacecraft to a desired target spacecraft. © Springer Science+Business Media B.V. 2009
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